This is a little circuit to measure current and voltage of a Raspberry Pi or other small computers using 5V input. In contrast to ready to use USB-dongles which you just plug in between the power source and the consumer, this circuit feeds it's measurement into the SPI-interface of a (second) Pi, therefore you can record current, voltage and power over time.

The software is feature complete (but maybe not bug-free).

The module designed and created by Lothar Hiller mainly uses two components

• an ADC converter with at least two channels (e.g. MCP3002)
• a Hall sensor

See the circuit for details, also available are a top and bottom view of the circuit:

• top view
• bottom view

The ADC will read the voltage on it's first channel directly. In the Hall sensor the current induces a magnetic field, which in turn induces some voltage which we feed into the ADC's second channel. We therefore measure the current indirectly.

The output-pins of the ADC are connected to the SPI interface of the pi.

Note that there is no sophisticated filtering of noise in hardware. The software takes care of this using oversampling.

The measurement software supports a LCD display with 4 rows and 20 columns attached to the I2C-interface of the Pi running the measurements.

The display is not strictly necessary, since it only displays live data. The recording of the data is independent of the display. When the measurement is started from a terminal (e.g. using ssh), the live data is also shown in the terminal.

To attach the LCD to the Pi, you at least need a parallel-to-serial adapter for the LCD. Since most of the LCDs are targeted towards the Arduino-platform, they need 5V. So in addition you also need a level-converter to convert the 3.3V of the Pi to the 5V of the display.

An alternative to the level converter is to manipulate the i2c-converter and to remove two resistors (labeled R8 and R9 in the above image). These resistors attach SDA and SCL to 5V. In the image you can see the R9 already removed and the R8 is partially off.

If unsure, you should read the tutorials on the net on how to use and attach an LCD to a Pi.

Optionally, you can attach a button and/or a LED to GPIOs. The installation defaults are GPIO23 for the button and GPIO18 for the LED. Using the button, you can start and stop recording data. During recording the LED will blink.

Please read the tutorials on the internet on how to attach a button and a LED to a Raspberry Pi.

This project contains a number of software-components for the measurement of the ADC values:

• vameter.py: This script reads the values of the ADC using the SPI interface and records the data in a round-robin-database (RRD-database)
• vameterctrl: A wrapper script for starting data-collection using a button attached to a GPIO-pin
• vameter-web.py: A small webserver for starting and stopping the data collection and for the presentation of the results.

The installation assumes that you have a freshly installed Raspbian-Stretch-Lite installation (Jessie won't work since the version of rrdtools/python-rrdtools is too old on Jessie).

Use the following commands to install the software:

sudo apt-get update
sudo apt-get -y install git
git clone https://github.com/bablokb/pi-vameter.git
cd pi-vameter.git
sudo tools/install
cd ..

git clone https://github.com/bablokb/gpio-poll-service.git
cd gpio-poll-service
sudo tools/install

The second set of installation commands is only necessary if you wish to use buttons attached to GPIOs to control the measurement. In this case you should check the file /etc/gpio-poll.conf. This file contains the configuration of the pin for the button.

The install command will copy all scripts, install additional software (mainly the required python-modules), modify /boot/config.txt, add a system-user for the web-service and add a number of system-services. Due to the changes to /boot/config.txt you have to reboot your system to activate the changes.

Besides the configuration of the GPIO for the start/stop-button you also have to set some basic configuration values in /etc/vameter.conf. The first variable is just a configuration. The other two are calibration variables necessary to deal with sample variation and effects of the layout of the module:

• ADC: the type of ADC you are using
• U_CC_2: the Hall-senser should output this value for I = 0mA
• CONV_VALUE: conversion-value of the Hall-sensor (see the datasheet)

Sadly, every ADC-converter needs it's special read commands and you have to configure the data for the ADC you are using. The script already contains some values for widely used ADCs (check variable ADC_VALUES at the beginning of the script). If your ADC is already supported you just have to change the variable ADC. Otherwise, you have to add the values of your ADC to ADC_VALUES first (and submit a patch).

The value of U_CC_2 has to be determined through measurement of Viout without any load - the Viout of the Hall-sensor should read something near 2.5. Run the following command for a few minutes:

vameter.py -V -O plain -S test.rrd

and take the average value of UI as U_CC_2.

The value of CONV_VALUE needs some more care. Usually the value from the datasheet (e.g. 0.185 V/A for the ACS712) is fine, but sometimes this has to be adapted. To estimate the value, measure a fixed load for a couple of minutes using vameter.py, check the average current and adapt the value as necessary:

1. attach a load resistor of e.g. R=100 Ohm
2. measure the voltage U with a digital multi-meter
3. measure I_DVM (using the digital multi-meter) or calculate I_DVM = U/R.
4. run vameter.py -r -S test.rrd for a couple of minutes
5. calculate fac = I_avg/I_DVM
6. replace CONV_VALUE in /etc/vameter.conf with fac*0.185.

Note that CONV_VALUE also depends on the temperature, see the datasheet of the ACS712 for details. So in theory, you need to calibrate the module before using it. But since this is not a high-quality scientifc device, a one-time calibration for a typical setup should be fine.

Data is stored in a round-robin-database using the rrdtools. The size of the database does not grow with measurement time, since only a limited amount of data is saved while older data is saved only in aggregated form. The vameter.py script saves one hour of data with seconds-resolution, 24 hours of data with minute-resolution and so on.

The command

vameter.py -h

will show a list of all available options. A typical use case is to start the data collection and create a set of graphs afterwards:

vameter.py -g IUP -r mydata.rrd

This will start recording measurements and save the results in file mydata.rrd. To stop the data-collection, hit CTRL-C. Since we passed the option -g IUP, the script will generate after termination three graphics for current, voltage and power .

You can use

vameter.py -p mydata.rrd

to print existing data and

vameter.py -S mydata.rrd

to print a summary of the results.

If you have attached a button to a GPIO and configured /etc/gpio-poll.conf accordingly, you can start and stop data collection with the button (no need to ssh to the system anymore).

Data is automatically saved in the directory /var/lib/vameter/data. The database name is auto-generated from the start date and time.

The project also installs small webserver using port 8026. If you point your browser to the address

http://ip-of-your-pi:8026/

you will see the webinterface of the system. The buttons on the top will let you start (and stop) the data collection or will let you shutdown the system.

The middle part shows all available measurements (from directory /var/lib/vameter/data) with some summary data. The bottom part will show a list of tabs to select a graphical representation of the current, voltage and power consumption during the measurement.